1. Field of the Invention
This invention relates to building fatigue resistant foundations for supporting columns, towers and structures under heavy cyclical loads such as for onshore and offshore wind turbine towers. Wind turbine support structures are subjected to high cyclical loading with the number of load cycles up to 109. Therefore, the fatigue design becomes more important for concrete construction and the influence of multi-stage and multi-axial fatigue loading have to be considered. Studies have recommended modification of the design rules for concrete construction of wind turbine foundations in order to consider the influence of multi-axial loading.
Wind turbine manufacturers have successfully developed large wind turbines with rated power ranging from 1.5 to 10 MW, for onshore and offshore installations. The E-126 model turbine by Enercon with a 7 MW rated power required a 29 meter diameter circular foundation with 1,400 cubic meters of concrete and 120 tons of rebar. The RePower 5M turbine with a 5 MW rated power required a 23 meter diameter circular foundation with 1,300 cubic meters of concrete and 180 tons of rebar. The task of building such large foundations is monumental and requires a great deal of construction planning and logistics. The proposed foundation designs and their associated construction methods provide cost-effective solutions for such challenging foundation projects.
Some wind turbine installations that have been constructed in the last 10 years in the US and Europe have encountered structural problems stemming from thermal cracking during construction, or from fatigue cracking requiring repairs. The present invention improves the geometry of the foundation in order to enhance dissipation conditions for the typical temperature rise due to heat of hydration after casting and also provides a cost effective fatigue resistant design.
2. Description of the Related Art
Conventional gravity style foundations for large wind turbines usually comprise a large, thick, horizontal, heavily reinforced cast in situ concrete base; and a vertical cast in situ cylindrical pedestal installed over the base. There are several problems typically encountered during the construction of such foundations.
Fatigue resistance of such conventional footings is achieved by over sizing the structural concrete elements and the reinforcing elements such that the resulting stress amplitudes are small enough for the structural elements to pass fatigue design checks.
The main problem is the monumental task of managing large continuous concrete pours, which require sophisticated planning and coordination in order to pour large amounts of concrete per footing, in one continuous pour, without having any cold joints within the pour.
Another problem is logistics coordinating with multiple local batch plants the delivery plan of the large number of concrete trucks to the job site in a timely and organized manner.
A further problem is the complexity of installing the rebar assembly into the foundation which requires assembling two layers of steel reinforcing meshes that are two to six feet apart across the full area of the foundation, while maintaining a strict geometric layout and specific spacing. This rebar assembly is made of extremely long and heavy rebar which requires the use of a crane in addition to multiple workers to install all the components of the assembly. The rebar often exceeds forty feet in length, thus requiring special oversized shipments which are very expensive and usually require special permits. The installation of the rebar is a labor intensive and time consuming task requiring a large number of well trained rebar placing workers.
Another important problem is the fact that the majority of the construction process consists of field work which can easily be compromised by weather conditions and other site conditions.
Another problem is thermal cracking of concrete due to overheating of the concrete mass. When concrete is cast in massive sections, the temperature can reach high levels and the risk of thermal cracking becomes very high. Thermal cracking often compromises the structural integrity of foundations as reported in many projects in Europe and North America.
Multi-cell caissons used in offshore installations always lack multi-axial post-tensioning elements and their fatigue resistance relies completely on heavily reinforced oversized concrete elements which involves expensive and labor intensive construction.